The three clusters were observed in the same night (July 3, 1998)
with the 1.5 m Danish telescope at ESO (La Silla).
We employed an EFOSC camera equipped with a Loral/Lesser CCD detector
C1W7 with
pixels. The
pixel size is 15
m, corresponding
to 0.39" on the sky, which provides a full field of
.
In Fig. 1a is shown a full field V 15 min exposure of Terzan 9.
Notice the remarkably patchy extinction in the region. We estimated
from the field stars variations of mag
between high and low
extinction patches. In Fig. 1b we show a cluster extraction
showing the core region.
In Fig. 2a we show a V image (3 min exposure) extraction for
NGC 6139 where the populous nature of the cluster is evident. Finally, in
Fig. 2b we provide a V extraction (1 min exposure) of NGC 6453.
![]() |
Figure 1:
V image extractions of Terzan 9:
a) whole field (![]() ![]() |
The log of observations is provided in Table 1.
Daophot II was used to extract the instrumental magnitudes. For calibrations we used stars from Landolt (1983) and Landolt (1992).
Reduction procedures in such reddened crowded fields were discussed in detail in a study of Liller 1 (Ortolani et al. (1996) and references therein). The equations for the present clusters are:
V = 24.24 + v
I = 23.09 + i
reduced to 1 s exposure time and 1.1 airmass. Due to crowding effects arising in the transfer of the aperture magnitudes from standards to the field stars, the zero point calibration errors are dominant, estimated to be about
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